net: Add GARP applicant-only participant

Add an implementation of the GARP (Generic Attribute Registration Protocol)
applicant-only participant. This will be used by the following patch to
add GVRP support to the VLAN code.

Signed-off-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/linux/netdevice.h

@@ -740,6 +740,8 @@ struct net_device
 	struct net_bridge_port	*br_port;
 	/* macvlan */
 	struct macvlan_port	*macvlan_port;
+	/* GARP */
+	struct garp_port	*garp_port;
 
 	/* class/net/name entry */
 	struct device		dev;